In order to form an electrostatic latent image, an electrophotographic image forming device electrically neutralizes the surface charge on a photoreceptor by exposing the photoreceptor according to image data. To achieve this, a photosensitive layer of the photoreceptor typically contains a charge generating substance which generates electric charges when irradiated with light, and a charge transporting substance which transports the generated electric charges.
It is desired that a photoreceptor with such a photosensitive layer allows for swift movement of electric charges by the charge transporting substance. In other words, the electric charges generated by the photosensitive layer as a result of exposure need to move to the photoreceptor surface and cancel the surface charge on the photoreceptor within a period from the charging of the photoreceptor to the development of the electrostatic latent image. If the surface charge is canceled only insufficiently, the exposed area does not make a good contrast in electric potential to the non-exposed area. A decrease in electric potential contrast will lead to a decrease in image density and render a white background appear gray.
Addressing these problems by specifying the mobility of electric charges generated in the photosensitive layer of the photoreceptor, patent documents 1 to 5 disclose image forming devices capable of producing high quality images. For example, patent documents 1 to 3 specify the electric charge mobility in the photosensitive layer in intense electric fields on the order of 105 V/cm.
Meanwhile, recent years have seen growing demand for development under low potential conditions, in order to achieve low power consumption in the developer device, reduced toner consumption, and improved durability for the photoreceptor, among others. To deliver quality development under low potential conditions, developing devices and developers are being improved (see, for example, patent documents 6 to 10). Patent document 10 discloses an image forming device which improves the durability of the photoreceptor by the use of a developer having a predetermined coloring ability to ensure a necessary post-fusion image density. The device can lower the amount of developer used per print.
(Patent Document 1)
Japanese patent 3227956 (registered Sep. 7, 2001), corresponding to U.S. Pat. No. 5,747,208.
(Patent Document 2)
Japanese publication of unexamined patent application 2000-305289 (Tokukai 2000-305289; published Nov. 2, 2000), corresponding to U.S. Pat. No. 6,521,386.
(Patent Document 3)
Japanese publication of unexamined patent application 2003-195536 (Tokukai 2003-195536; published Jul. 9, 2003)
(Patent Document 4)
Japanese patent 2833222 (registered Oct. 2, 1998)
(Patent Document 5)
Japanese publication of unexamined patent application 2001-324825 (Tokukai 2001-324825; published Nov. 22, 2001)
(Patent Document 6)
Japanese publication of unexamined patent application 10-83120 (Tokukaihei 10-83120/1998; published Mar. 31,1998)
(Patent Document 7)
Japanese publication of unexamined patent application 2003-29527 (Tokukai 2003-29527; published Jan. 31, 2003)
(Patent Document 8)
Japanese publication of unexamined patent application 2003-43783 (Tokukai 2003-43783; published on Feb. 14, 2003)
(Patent Document 9)
Japanese publication of unexamined patent application 2003-167441 (Tokukai 2003-167441; published on Jun. 13, 2003)
(Patent Document 10)
Japanese publication of unexamined patent application 2000-122355 (Tokukai 2000-122355; published on Apr. 28, 2000), corresponding to U.S. Pat. No. 6,122,468.
Patent documents 1 to 10 are silent about photoreceptors suitable for use in development under low potential conditions.
Electric charge mobility in a photoreceptor generally varies with an electric field. As such, the mobility of charge in the photoreceptor decreases under low potential conditions where the photoreceptor is charged only to a low potential. More specifically, electric charge mobility can be so low under low potential conditions that the electric charges generated in the photosensitive layer may not sufficiently move within a period from the exposure of the photoreceptor to the development. If the development is carried out with the exposed area remaining at a high potential, the exposed area does not make a good contrast in electric potential to the non-exposed area. A decrease in electric potential contrast will lead to a decrease in image density and render a white background appear gray. In order to deliver quality image production under low potential conditions, the electric charge mobility in the photosensitive layer in the photoreceptor needs be specified under low potential conditions.